docs: def add_key_filter(self, *args):
          """
          Add a single key filter to the inputs.
  
          :param args: a filter
          :type args: list
          :rtype: :class:`RiakMapReduce`
          """
          if self._input_mode == 'query':
              raise ValueError('Key filters are not supported in a query.')
  
          self._key_filters.append(args)
          return self